1. Field of the Invention
The present invention relates to a WDM (Wavelength Division Multiplexed) transmission system, a central controller for the system, and a method for controlling preemphasis in the system.
2. Description of the Related Art
Recently, an increased number of WDM transmission apparatuses are being installed within networks for service providers in response to drastic growth in requirements for communication lines, such as the Internet. As a result of the spread of WDM transmission apparatuses, there are demands for proper methods to simplify the optimization and the management of the apparatuses.
In a WDM transmission system, an optical transmitter transmits a WDM signal, into which a plurality of optical signals at respective different wavelengths are multiplexed, is transmitted to an optical receiver via one or more optical repeaters (optical amplifiers), which relay the WDM signal, arranged on the optical transmission line. Individual optical signals in the WDM signal may be at different intensities when received by the optical receiver because of the character of the optical transmission lines and a gain-wavelength dependence of each of the optical repeaters (i.e., an occurrence of a tilt) whereupon qualities (OSNR: Optical Signal to Noise Rate) of the optical signals as received at the optical receiver may be not equal.
In order to eliminate the possibility of an occurrence of a tilt, a technique of “preemphasis,” in which optical signals to be transmitted from an optical terminal have respective sufficient intensities that are previously set in such a manner that an optical receiver receives optical signals identical in intensity, is known to the art. An optimization of a WDM signal by a preemphasis technique requires, upon installation of a WDM transmission apparatus and/or prior to starting its operation, manual setting and changing of intensities of optical signals in the WDM signal, by which intensities an optical transmitter transmits the optical signals, based on the result of a measurement of intensities (a received OSNR) of optical signals received by an optical receiver.
However, this conventional preemphasis tends to fail in flexibility to respond to a change in one or more variation factors that causes a new setting for preemphasis (e.g., amounts of preemphasis) due to factors, such as, deterioration of the apparatus over time, change in the number of wavelengths used for optical signals (multiplexed into a WDM signal), or an error arising in a WDM transmission system.
For this reason, it is preferably suggested that at least one WDM apparatus on an optical transmission line in a WDM transmission network functions to monitor and control the above-mentioned variation factors at all times so as to autonomously respond to change in the variation factors. Nevertheless, such a WDM network is expensive and requires a large load to operate.